1. Field of the Invention
The present invention relates to a misfire diagnostic system suitable for use in internal combustion engines, and specifically to an engine-misfire diagnostic system employing an engine crank-shaft angle sensor, often called "crank angle sensor" which often takes the form of a reference-mark pulse generator.
2. Description of the Prior Art
In recent years, there have been proposed and developed various engine misfire diagnostic systems a high-precision misfire diagnosis. It is very important to more precisely make an engine-misfire diagnosis in order to accurately perform injection- and ignition-timing control. Many conventional misfire diagnostic systems usually utilize a so-called crank angle sensor for monitoring or detecting a combusting condition of each individual engine cylinder. Turning power, produced during combustion stroke at each engine cylinder, is correlated to a change in angular velocity of the engine crankshaft. The combusting conditions of individual cylinders, and the changes in angular velocity of the crankshaft are correlated to each other. A typical crank-shaft angle sensing device includes a reference pulse generator, called a "reference sensor", and/or an angular-position pulse generator, called, "position sensor". A typical reference sensor is designed to generate a reference pulse signal at a predetermined timing (for example, at 90 degrees of crankshaft rotation before the top dead center (T.D.C.)) every predetermined crank angles such as 720.degree./n equal to 180.degree. in case of a four-cylinder engine, where n denotes the number of engine cylinders, A typical position sensor is designed to generate a unit-angle pulse signal every unit crank angle such as 1.degree. or 2.degree. and also is provided for detecting an engine revolution speed. In some older-model cars, a photoelectric crank-shaft angle sensor is used. Many earlier-model cars use a magnetic crank-shaft angle sensor that takes a magnetic pulse pickup. Japanese Utility-Model Provisional Publication No. 5-17172 discloses an ignition-timing control system for an internal combustion engine in which fluctuations or variations in time duration of the reference pulse signal generated by the reference sensor are used for determining the presence or absence of a misfire, which may occur continually at a particular cylinder. In Japanese Utility-Model Provisional Publication No. 5-17172, an actual ignition timing of a cylinder, which cylinder is assigned to a combustion stroke executed in accordance with a predetermined firing order after the misfiring cylinder, is slightly retarded by a correction time based on both an engine revolution speed and a reference fuel-injection amount when the misfire is detected at the particular cylinder on the basis of the variations in the time duration of the reference pulse signals from the reference sensor. In other words, in the ignition-timing control system of Japanese Utility-Model Provisional Publication No. 5-17172, the presence or absence of engine misfire is determined on the basis of the fundamental concept that the combusting conditions of individual engine, cylinders are analogous with changes in angular velocity of the crankshaft (or with fluctuations in the time duration of the reference signal). However, even when the engine operating conditions are stable or steady without any engine misfires, the time duration (or time interval or time period) of the reference signal from the reference pulse signal generator, which duration is correlated to a combusting condition of each individual engine cylinder, can be affected by different factors, namely deviations in mechanical machining accuracy of a ring gear constructing a part of the crank-shaft angle sensing device, electrical deviations of input circuitry of a magnetic pickup, and/or irregularities in combustion among individual engine cylinders. It is desirable to eliminate these different factors to obtain correct or precise engine-misfire diagnosis. To filter out mechanical errors, i.e., variations in the reference-signal time interval, resulting from such deviations in mechanical machining accuracy or mechanical unevenness of the crank-shaft angle sensor, Japanese Patent Provisional Publication No. 7-197845 teaches the learning of mechanical errors included in the reference-signal time interval for a predetermined time interval of reference signals corresponding to 720 degrees of crankshaft revolution, when the engine is being cranked or during fuel-cut. However, irregularities in combustion among individual engine cylinders still remains. As may be appreciated, such irregularities in combustion among individual engine cylinders are different depending on engine operating conditions, namely engine speed and load. A learning value (learn value) obtained under only a particular condition is inadequate to make a high-precision misfire diagnosis.